The folllowing tutorial will give you a step by step process to follow for creating Joomla category in Joomla administrator.
Joomla Category Creation
Step1: First, you need to login to your Joomla admin-side. Then go to "Content-> Category Manager-> Add New Category"
Step2: Add your category title in "Title" field
Step3: If you want to use your own alias then put alias title like this way(eg: how-to-create-category-in-joomla3). Alias title always in lower-case letter. If you don't want to use your own alias then just leave it and then the alias title would automatically generate from your "Category Title"
Step4: Put your category content or descriptions
Step5: If you would like to create this category under any other parent category then select parent category from "Parent" options
Step6: Finally, click on save button from top left to save your new category